Malaria is a severe disease that ranks among the most prevalent infections in tropical areas throughout the world. Approximately 300-500 million people become infected yearly, with relatively high rates of morbidity and mortality. Severe morbidity and mortality occur particularly in young children and in adults migrating to a malaria endemic area without having undergone prior malaria exposure. The World Health Organization (WHO) estimates that 2-3 million children die of malaria in Africa alone, every year. The widespread occurrence and the increasing incidence of malaria in many countries, caused by drug-resistant parasites (Plasmodium falciparum, recently also Plasmodium vivax) and insecticide-resistant vectors (Anopheles mosquitoes), underscore the need for developing new methods for the control of this disease (Nussenzweig and Long 1994).
Malaria parasites have a complicated life cycle consisting of pre-erythrocytic, erythrocytic and sexual parasitic forms, representing a potential target for the development of a malaria vaccine. The pre-erythrocytic and erythrocytic forms are found in the host, while the sexual forms occur in the vector. Immunization with live-attenuated irradiated sporozoites (IrSp) has been shown to induce sterile protection (i.e., complete resistance against parasite challenge) in mice (Nussenzweig et al. 1967), non-human primates (Gwadz et al. 1979) and human (Clyde et al. 1973, Edelman et al. 1993). Protection conferred by IrSp is mediated by sporozoite neutralization by both humoral (B cell) and cellular (T cell) immune responses (Tsuji et al. 2001). Although an IrSp vaccination is an attractive solution, the only way to obtain sporozoites is by dissecting mosquito salivary glands, and there is currently no known technology to grow large numbers of sporozoites in vitro. Therefore, an alternate vaccine vector that can elicit an equally strong protective immunity against malaria is needed.
One promising target for such a vaccine vector is the circumsporozoite (CS) protein, which is expressed on the surface of the sporozoite. Effective neutralizing antibodies are directed against the immunodominant, species specific, repeat domains of the circumsporozoite (CS) protein. In Plasmodium falciparum (human malaria parasite), the repeats (NANP)n are conserved among isolates from all areas of the world. This central repeat contains multiple repeat of B cell epitopes, and, therefore, the CS protein can induce a strong humoral immune response by triggering B cells (Tsuji et al. 2001). At the C-terminal region of the CS protein, there are several T cell epitopes, which can induce a significant cellular immune response (Tsuji et al. 2001). The humoral (antibody) response can eliminate parasites by interacting and neutralizing the infectivity of sporozoites (extra-cellular parasite) prior to entering hepatocyte, whereas the cellular (T cell) response can attack EEF (an intra-cellular parasite) by secreting interferon-gamma. These immune responses prevent the EEFs from maturing and dividing rapidly to form thousands of merozoites that reenter the blood and infect erythrocytes causing the disease we recognize as malaria.
One CS-based malarial vaccine that is currently undergoing human trials is GlaxoSmithKline's RTS, S, fusion protein of the Hepatitis B surface antigen and a portion of Plasmodium falciparum circumsporozoite protein (PfCSP) in a form of virus-like particle (International Patent Application No. PCT/EP1992/002591 to SmithKline Beecham Biologicals S.A., filed Nov. 11, 1992), has been shown to decrease malaria infection in clinical trials (Alonso et al. 2004, Alonso et al. 2005, Bejon et al. 2008). RTS, S induces an anti-PfCSP humoral immune response, but a relatively weak PfCSP-specific cellular (CD8+) response (Kester et al. 2008), which might be the reason for the relatively weak protection by RTS, S. In contrast, adenovirus-based malaria vaccines can induce a protective cellular immune response (International Patent Application No. PCT/EP2003/051019, filed Dec. 16, 2003, Rodrigues et al. 1997). However, there are currently two obstacles that limit the use of an adenovirus-based platform as a malaria vaccine: (1) lack of a capability of inducing a potent humoral response against a transgene product, and (2) pre-existing immunity to adenovirus, especially adenovirus serotype 5, which hampers the immunogenicity of adenovirus-based vaccine.
One approach that has recently been taken in an attempt to augment adenovirus-induced humoral response is to insert a B cell antigenic epitope (e.g., a bacterial or viral epitope) in adenovirus capsid proteins such as Hexon, Fiber, Penton and pIX (Worgall et al. 2005, McConnell et al. 2006, Krause et al. 2006, Worgall et al. 2007).
In addition, to circumvent pre-existing immunity to adenovirus serotype 5 (Ad5), other adenovirus serotypes with lower seroprevalence, such as adenovirus serotype 11, 35, 26, 48, 49 and 50, have been evaluated as a vaccine platform and shown to induce immune response to a transgene in spite of the presence of anti-Ad5 immunity (International Patent Application No. PCT/EP2005/055183 to Crucell Holland B.V., filed Oct. 12, 2005, Abbink et al. 2007). Substitution of Ad5 Hexon, which is the target capsid protein of neutralizing antibody, with that of other serotypes has also been constructed in order to escape pre-existing anti-Ad5 immunity (Wu et al. 2002, Roberts et al. 2006).
There is, however, no improved adenoviral vector reported to have overcome the two obstacles at the same time in applying an adenoviral vector to a malaria vaccine mentioned above. Given that seroprevalence to Ad5 is high in malaria endemic areas (Ophorst et al. 2006.), there is a need for an adenovirus-based malaria vaccine that induces both protective humoral and cellular immune responses even in the presence of pre-exiting immunity to adenovirus.